1. Field of Invention
The present invention relates to a sliding bearing, more particularly to a sliding bearing which comprises; a bearing-alloy layer referred to as the lining and consisting of copper-lead alloy or an aluminum alloy; and an overlay, which consists of a soft-alloy plating layer and is deposited on the lining to impart compatibility; and a Ni plating barrier layer and the like occasionally formed between the lining and the overlay. The present invention also relates to a method for producing the sliding bearing.
2. Description of Related Art
The sliding bearing described above is used mainly for the journal portion of a crankshaft, or the big end of a connecting rod of an internal combustion engine. Lead alloys are mainly used as the overlay and, occasionally, tin alloys.
One of the present applicants has succeeded in improving the composition of the lead overlay alloy as seen in German Patent No. 3000379 and improving the crystal orientation as seen in Japanese Unexamined Patent Publication (kokai) No. 8-20893.
Since lead is an environmental pollutant, discontinuance of its use or reduction in the amount of its use is requied. A development for avoiding the use of lead as the overlay is directed towards bonding such tribological material as MoS.sub.2 with resin to form the overlay film. In addition, one of the present applications proposed in European Patent Publication No. 0795693A2 a Cu--Ag alloy which may not necessitate an overlay.
Incidentally, bismuth is a low-melting point metal as lead is. Bismuth is harder and more brittle than lead. Specifically, the hardness Hv.sub.0.2 of lead is 5, while the hardness Hv.sub.0.2 of bismuth is 10. The electro-plating, which is frequency used in the formation of an overlay, hardens the resultant layer due to absorption of hydrogen. That is, the hardness Hv.sub.0.2 of electro-plated lead is 10, while the hardness Hv.sub.0.2 of conventionally electro-plated bismuth is approximately 20. Such property of bismuth is inappropriate to attain fatigue resistance and compatibility of the sliding material. Bismuth has, therefore, not been used for the sliding material.
Meanwhile, the low-melting property of bismuth is utilized in the bismuth-based soldering alloy or the bismuth-based core of a mold. Bismuth is also used for semiconductor material, electronic material, optical recording media (for example, Japanese Unexamined Patent Publication No. 4-51742), and magnet material (Mn--Bi magnet). This recording medium is prepared by sputtering.
The present inventors made extensive researches into the replacement of lead with bismuth, in an attempt to utilize the better corrosion resistance of bismuth in acidic solution than that of lead and to avoid the toxicity of lead.